This may or may not be also related to the computer CPU story i read a while back. I'm at work, so no time to look up the entire story, but it goes something like this:
intel designed and made the 8086, 80286, 80386, 80486 processors. AMD at the time was a 'second supplier', meaning they made them, using intel's designs, and paid a license fee. Then for various reasons, intel decided that it didn't want AMD making them anymore, it viewed them more as a competitor than a second supplier. Big massive court case, intel claimed that amd was only allowed to make up to the 386, amd said the license should also cover 486 because it includes the 386 instruction set, intel said they had a trademark on the name 486.
and that's the point i'm getting to, in that at the end it was decided that (at least in the US, where they were doing battle) you can't trademark a number.
and so at the end of this story, the name 'pentium' was born, intel could trademark it, stop AMD making anything they designed, and AMD had to come out with their duron/sempron/athlon etc names.
So maybe there's also a parallel here to cameras? By this logic, if canon brought out the 'eos 500' in the US, then they have the trademark on 'eos' of course, but there is legally nothing to stop any other company coming out with a camera called '500' (seriously, who ever says "i have an eos 500"?). i know they sell 1d/ds/5d in the US, maybe it's got to do with how many letters vs numbers are in the name?
and the rebel/kiss names have been around for over 20 years, film cameras were also called rebels, and the names just carried over...